Power Systems
A buchholz relay is used for

 protection of a transformer against all external faults
 both 1 and 2
 nether 1 nor 2
 protection of a transformer against all internal faults

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Power Systems
The value of off diagonal elements is

 which is connected between bus i and bus j with positive sign
 which is connected between bus i and bus j with negative sign
 sum of admittances connected at bus j
 sum of admittances connected at bus i

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